The invention relates generally to the preparation of an image of an original.
More particularly, the invention relates to the automatic correction of color casts during electronic image processing.
In the automatic correction of color casts, an original is electrooptically scanned in the three primary colors at a multiplicity of points arranged in a series of columns and rows. The resulting density signals are averaged over groups of the points to yield a reduced number of average density signals which represent an image of decreased resolution. The average density signals are subsequently transformed into chrominance signals, i.e., color difference signals, and luminance signals in such a manner that a luminance signal and a pair of chrominance signals are associated with each group of points.
Electronic image processing using color correction circuits is being increasingly employed for the reproduction of colored originals (positive-positive) and for the production of colored photographic positives from colored negatives. See, for example, the West German Publication No. 36 29 469 and the literature cited therein. As noted above, the original is scanned at a multiplicity of points defining a series of columns and rows. Scanning of the original normally occurs serially in the three primary colors red, green and blue. The resulting electrical signals, which represent an image of the original, are then modified or corrected in accordance with specific criteria. As a rule, the signals are digitized and can thereafter be temporarily stored in digital memories. The modified signals are sent to a color exposure unit, e.g., a CRT scanner-printer system, which operates on points of the image serially. The exposure unit converts the electrical signals into an optical image and recording material such as, for instance, color negative paper, is subsequently exposed to the image. It is noteworthy that the optical image is produced point-by-point through transformation of the electrical image signals.
Transformation into luminance and chrominance signals is known from the video art. Here, color corrections are performed by electronic means where there are aberrations in color, e.g., color casts, resulting from photography or where color falsification occurs during transfer and must be compensated for.
In the production of color photographic images using a color image reproduction system, undesired deviations in color are conventionally compensated for by adjusting the average value of the color vectors of all image points to the standard gray point. To this end, a color balance regulator is provided in practice and functions to effect a shift in color vector, for example, by increasing the amplification in one or two color channels. It is also possible to produce predetermined deviations from the standard gray point when the motif of the original makes this desirable, e.g., green lawns or red evening sky. Since an average is taken over the entire image, only overall compensation for or correction of color deviations or color casts can be achieved. Frequently, however, different color casts are present in areas of an image having different luminances. Thus, in strongly illuminated portions of an image, there can be a shift towards red (a red cast), for instance, while more weakly illuminated portions have a cyan cast. This type of appearance is referred to as "color imbalance". Such color casts cannot be compensated for with the above-described, overall color balance adjustment or regulation.